The invention relates to telecommunication line termination equipment and in particular to Integrated Services Digital Network (ISDN) line termination equipment.
There is an ever pressing need to provide consumers of telecommunication services with high speed data services. Many subscribers use plain old telephone services (POTS) lines to carry data communication signals between computer devices employing modems to communicate via the public switched telephonic network. Unfortunately, the maximum data rate using modems of a POTS subscriber line is limited by many factors including the sampling rate, bandwidth, and the analog to digital signal conversion rate for the line cards which terminate the POTS lines.
Other subscriber lines are available, however, which provide higher data rates. Integrated Services Digital Network (ISDN) lines provide a subscriber with telephone services generally at a higher data rate than conventional POTS lines. Subscribers utilize an ISDN line to carry digital signals controlled by a personal computer to another ISDN set, modem or ISDN modem pool via the public switched telephone network. It will be appreciated that the maximum data rate which can be transmitted using modems over an ISDN subscriber line is limited to 144 kilobits per second.
Subscribers can lease special lines from their telephone providers which accommodate higher data rates than are supported by conventional dial-up subscriber lines or by ISDN lines. Recently, Asymmetric Digital Subscriber Line (ADSL) technology has evolved which provide even higher data transmission speeds than ISDN technology. The known ADSL technique which is supported by available equipment provides a significantly higher data rate to a subscriber over a conventional copper two-wire subscriber line. Proper terminating equipment at the consumer location and the telephone service provider central office is required to employ ADSL technology.
Referring now to FIG. 1, currently available services for subscribers using conventional two wire copper loops are illustrated. Equipment to the right of dashed line 10 represents customer premises equipment (CPE); equipment to the left of line 10 represents central office line termination equipment. The ISDN line interface 12 also known as a line card, provides an interface between digital incoming and outgoing communication lines 14 and 16, and analog signals carried on subscriber line 18. For example, lines 14 and 16 may carry 2-64 kilobits per second (kbps) and 1-16 kbps signals representative of information received from and transmitted to line 18. A main distribution frame (MDF) 20 is used to interconnect a plurality of incoming subscriber lines to various terminating equipment at the central office. In this example, line 18 is connected through an ISDN splitter 19 and MDF 20 to subscriber line 22. A conventional ISDN station set or instrument 24 at the consumer""s premises is connected through ISDN splitter 23 to subscriber line 22.
FIG. 1 also illustrates another service to the subscriber which provides a high speed data capability. An ADSL interface circuit 26 provides an interface between the central office and the subscriber for transmitting and receiving data at rates up to several Megabits per second. Lines 28 and 30 provide inbound and outgoing digital data communications representative of information to and from the subscriber carried on line 32. Line 32 is connected via ISDN splitter 19 and MDF 20 to subscriber line 22. An ADSL interface 36 provides an interface between conventional digital data communicated with a user""s personal computer 38 and ADSL analog format signaling communicated on line 37. The ISDN splitter 23 couples the ADSL signal between ADSL interface 36 and subscriber line 22. The advantage to the user is that the ADSL facilities support a substantially higher data rate than would be available if the subscriber utilized communications terminated via the ISDN line interface 12.
A disadvantage illustrated in FIG. 1 is that ISDN splitters 19 and 23 are required. These known ISDN splitters function to separate the higher frequency signals associated with ADSL signaling from the lower frequency signals (typically  less than 80-110 kilohertz) associated with the conventional ISDN communications. The conventional ISDN splitter consist of a lattice of inductors and capacitors that provides the needed filtering while maintaining the ISDN signals. The components (inductors and capacitors) used in the ISDN splitter occupy a relatively large volume. Thus, a conventional ISDN splitter occupies a significant amount of space.
Unfortunately, conventional ISDN termination equipment is complex (especially in wiring devices) and is not highly integrated when employing ADSL technology. Moreover, bulky splitters are required taking up a significant amount of space in the terminating equipment cabinets. The size of the splitters and the wiring associated with connecting them to the MDF, ADSL interface and the ISDN line interface take up precious space which is limited. The problem is magnified as more and additional subscribers request services. Accordingly, there is a need to provide highly integrated equipment enabling high speed ADSL technology services on ISDN lines while minimizing space and wiring requirements.
It is an object of the present invention to address the above referenced need by providing a solution which minimizes the space and wiring associated with the central office terminating equipment for high speed data services.
In accordance with one embodiment of the present invention, access interface units terminate subscriber lines by which the corresponding subscribers can utilize ISDN services and/or high speed data services such as ADSL. The access interface units support both types of service without utilizing a conventional ISDN splitter. The access interface units include a line support circuit wherein two way communications on the subscriber loop are separated into inbound and outbound communications on separate channels. The inbound communications are split into first and second signals. The first signal is low pass filtered and converted into digital format signals by a conventional ISDN receiver. The second signal is high pass filtered and converted into digital format signals by a high speed data receiver. The outbound communication is the summation of third and fourth analog signals from an ISDN transmitter and high speed data transmitter. An important aspect of this invention resides in the ability to use conventional low voltage, low current passive and active components for the low pass and high pass filters.